1. Field of the Invention
The present invention relates to a magnetic disk apparatus having a head-crash preventing function.
2. Description of the Prior Art
In magnetic disk apparatus, write and read of information is performed by floating the head above the disk holding a narrow space therebetween by the principle of dynamic pressure bearing. In order to improve the read/write characteristics, this spacing is required to be a minute value equal to or less than 0.5 .mu.m. On the other hand, if the spacing is made small, there is an increasing possibility of occurence of the head-crash accident in which the disk and the head are brought into contact with each other and the recording medium and the head are damaged.
In this instance, in the magnetic disk apparatus, generally, disks are rotated as an assembly which is assembled by stacking several sheets of disks. On the surface of each disk, there are provided one to two heads, and these heads (amounts to a total of 10 to 20) are simultaneously positioned at respective corresponding tracks by a positioning device and perform read/write operations. However, among these heads, it is one head which performs the read/write operation at a time.
Further, a read/write command is delivered from a computer several times in one second per one positioning device (that is per 10 to 20 heads). Still further, one time of read/write operation finishes in less than 50 ms even including a positioning time. In short, a time period in which one head is performing the read/write operation in the working time of the disk apparatus is relatively very short.
Nevertheless, in conventional disk apparatus, all the heads are held always in a floating condition with a narrow space, and hence the possibility of head-crash has been increasing undesirably.
There are floppy disk apparatus as memory units in which the distance between the head and disk surface is changed in the read/write operation. However, in the floppy disk apparatus, the head is held in a contact condition with the disk while the disk is rotating, and consequently wear is caused in both the disk and the head. To cope with such a situation, a measure is employed to separate the head from the recording medium by the use of a solenoid or the like when the read/write operation is not conducted. However, in usual disk apparatus of a large type, data processing speed which is much faster (about 1/10 in time) than that for the floppy is required, and thus if this measure in the floppy disk apparatus is applied, there have been problems in which it takes an excessively long time for the head to be in a floating condition again, and in which there is an increasing possibility of damaging the disk on the contrary, due to a contact of the end of the head with the disk.